{"id":510,"date":"2017-06-04T23:18:48","date_gmt":"2017-06-05T07:18:48","guid":{"rendered":"http:\/\/wonghoi.humgar.com\/blog\/?p=510"},"modified":"2017-06-04T23:26:55","modified_gmt":"2017-06-05T07:26:55","slug":"tds-500-600-700-800-series-monochrome-crt-horizontal-linearity","status":"publish","type":"post","link":"https:\/\/wonghoi.humgar.com\/blog\/2017\/06\/04\/tds-500-600-700-800-series-monochrome-crt-horizontal-linearity\/","title":{"rendered":"TDS 500 600 Series (and TDS 820) Monochrome CRT Horizontal Linearity"},"content":{"rendered":"

I have a batch of TDS 620A which everything looks shiny new, but the display will\u00a0stretch vertically and eventually gone unstable after turning it on continuously for a long time.<\/p>\n

Turns out\u00a0there’s a specific batch of flyback transformer (120-1841-00) on the monochrome CRT driver board (640-0071-06) that has quite a bit of infant mortality. The newer the unit looks (of the CRT tube looks shiny new without burn-in), the more likely it’s a victim of the bad batch.<\/p>\n

The transformer is almost impossible to source (other than getting another CRT driver board), but I was able to find a Chinese supplier who makes it.\u00a0It was usable, but it’s a nightmare to get it on because it’s really done\u00a0with\u00a0the stereotypical Chinese (PRC) manufacturing caliber. \u00a0When I received the unit, it’s a WTF moment!\u00a0Leave me a comment if you want me to write about it.<\/p>\n

The replacement transformer is only pin and functionality compatible, but it’s not a drop in replacement (not even geometrically). The characteristics are different and I had to adjust the trimmers\u00a0all over the place.<\/p>\n

I was able to get the screen width I want by adjusting the variable inductor (L105, HOR SIZE) by nearly pulling ferrite rod out, but the horizontal linearity was way off (the left side is very squeezed):<\/p>\n

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I looked into the TDS520B Component Service Manual (Same CRT board circuit diagram) and found this:<\/p>\n

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But L100 looks like this, which doesn’t seems trimmable:<\/p>\n

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The right hand side is the same L100 choke I extracted from a CRT driver board (same model) that I’ve disposed of.\u00a0I saw two suspicious\u00a0pieces of metal-like objects strapped on the choke on the left (installed) which I haven’t seen in other identical boards.\u00a0<\/p>\n

Thinking that by changing the magnetic property of the core, I can adjust the inductance of an otherwise non-adjustable inductor. I took a few bits of magnets sitting on my bench and swing it around the L100 choke, the\u00a0horizontal display widens\/narrows depending on which pole of the magnet is facing the L100 choke.<\/p>\n

After a few trial and error, I picked the right amount of\u00a0magnet discs to correct the horizontal linearity so the squares\u00a0have roughly the same width:<\/p>\n

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I guess I cracked the code! Here’s the result of\u00a0correction by magnet:<\/p>\n

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I have a batch of TDS 620A which everything looks shiny new, but the display will\u00a0stretch vertically and eventually gone unstable after turning it on continuously for a long time. Turns out\u00a0there’s a specific batch of flyback transformer (120-1841-00) on … Continue reading →<\/span><\/a><\/p>\n

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